<!DOCTYPE HTML>
<html>

<head>
  <meta charset="utf-8">
  <style>
    body {
      height: 2000px;
      /* make body scrollable, the tooltip should work after the scroll */
    }

    .tooltip {
      position: fixed;
      padding: 10px 20px;
      border: 1px solid #b3c9ce;
      border-radius: 4px;
      text-align: center;
      font: italic 14px/1.3 sans-serif;
      color: #333;
      background: #fff;
      box-shadow: 3px 3px 3px rgba(0, 0, 0, .3);
    }
  </style>
</head>

<body>

  <p>LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa</p>
  <p>LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa LaLaLa</p>

  <button data-tooltip="the tooltip is longer than the element">Short button</button>
  <button data-tooltip="HTML<br>tooltip">One more button</button>

  <p>Scroll the page to make buttons appear on the top, check if the tooltips show up correctly.</p>


  <script>
    let tooltipElem;

    document.onmouseover = function(event) {
      let target = event.target;

      // if we have tooltip HTML...
      let tooltipHtml = target.dataset.tooltip;
      if (!tooltipHtml) return;

      // ...create the tooltip element

      tooltipElem = document.createElement('div');
      tooltipElem.className = 'tooltip';
      tooltipElem.innerHTML = tooltipHtml;
      document.body.append(tooltipElem);

      // position it above the annotated element (top-center)
      let coords = target.getBoundingClientRect();

      let left = coords.left + (target.offsetWidth - tooltipElem.offsetWidth) / 2;
      if (left < 0) left = 0; // don't cross the left window edge

      let top = coords.top - tooltipElem.offsetHeight - 5;
      if (top < 0) { // if crossing the top window edge, show below instead
        top = coords.top + target.offsetHeight + 5;
      }

      tooltipElem.style.left = left + 'px';
      tooltipElem.style.top = top + 'px';
    };

    document.onmouseout = function(e) {

      if (tooltipElem) {
        tooltipElem.remove();
        tooltipElem = null;
      }

    };
  </script>

</body>
</html>
